WO1985004596A1

WO1985004596A1 - Process for recovering lost heat from combustion gases

Info

Publication number: WO1985004596A1
Application number: PCT/EP1984/000112
Authority: WO
Inventors: Franz Joseph Gattys
Original assignee: Gattys Technique S.A.
Priority date: 1983-03-19
Filing date: 1984-04-11
Publication date: 1985-10-24
Also published as: EP0180575A1; JPS61501895A; DE3310014A1

Abstract

In this process it is provided in a first step to transmit heat by direct contact of the gas with a magnesium hydroxide suspension Mg(OH)2 having a controlled pH value comprised between 4.5 and 5.5, and in a following step to effect an indirect heat transfer from the suspension to a heat-conveying fluid. The adjustement of the pH value is carried out by means of sodium dihydrogensulfate (NaH2PO4) and Glauber salt is used as heat conveying fluid. There is continuously added to the combustion gas, before it reaches the absorption stage, nitrogen dioxide which corresponds to the stoechiometric amount of nitrogen dioxide in the top portion of the absorption column. The portion removed from said suspension and subjected to retreatment is concentrated at 40% by weight and is alkalised with ammonia, whereafter the magnesium hydroxide is separated by filtration and the residual solution is prepared as fertilizer. The phosphate fraction is recycled.

Description

Process for the recovery of waste heat from the flue gases of incineration plants

The invention relates to a method for recovering waste heat from flue gases from combustion plants.

As is well known in the economy, there is a large amount of low-temperature heat at temperatures between 100 and 150 ° C, the use of which fails, above all, the lack of suitable options for storage and transport from the place of origin to the consumer. So it can be assumed, for example, in the operation of conn nections that depending on the exhaust gas temperature between 10 and 15% of the heat energy - coal or oil - brought with the heating energy is released as waste heat into the atmosphere, so only in one in the Time perspective foreseeable coal consumption 100 million tce for electricity generation in the Federal Republic with e energy loss of between 10 and 15 million tce as low temperature w can be expected. Taking into account all other ELnL ^' s, which are effective, for example, in thermal production processes, it can be assumed that the use of the primary energy used significantly exceeds 40% n, ie that approximately 60% of the primary energy used is dissipated heat loss into the environment. On the other hand, just over 40% of the energy consumed in the Federal Republic requires low-temperature heat for household purposes. In addition, exhaust gas cleaning, in particular the exemption of the exhaust gases from the Combustion of fossil fuels sulfur dioxide u

Nitrogen oxide impurities are still unsatisfactory

Problem. So even after taking into account the current technical possibilities, such as the use of fluidized bed furnaces and the use of known desulfurization processes - without targeted flue gas desulfurization - with a sulfur emission of about 2.8 to 3.5 S / kg SKE in the form of sulfur dioxide become.

The aim of the present invention is to provide a method, the help of which low-temperature heat, in particular the waste water from flue gases, can be recovered in a simple manner and stored in a manner that makes it possible to transport it. The invention consists in that the heat contained in the flue gas is first in oinor first stage by direct contact with a magnesium hydroxide suspension (Mg (0H) 2) adjusted to a pH value of 4.5 to 5.5 and kept at the pH value then a heat exchanger stage is indirectly transferred to a heat transfer medium.

The invention provides a method by means of which low temperature heat can be recovered in a simple manner and saved in a form which enables easy transport even over longer distances and can be called up as required. Through the use of magnesium hydroxide as an intermediate medium for the transfer of heat direct contact, a practically quantitative cleaning of flue gases from their sulfur dioxide and slick oxide impurities and chemical bonding is achieved at the same time, due to the pH adjustment to a value between 4.5 and 5.5 a blocking of the magnesium hydroxide Absorption of the carbon dioxide contained in the flue gas is prevented. The flue gases treated in the manner according to the invention not only reach the atmosphere largely free of unused heat, but also almost pollutant-free.

In an advantageous embodiment of the invention, the pH is adjusted with the aid of sodium dihydrogen sulfate (NaH2P04), the magnesium hydroxide suspension being added in an amount of 140 to 180, advantageously up to 160 g / l. This addition of a st ionizing salt, on the one hand, as an advantage for the process control, results in a considerable improvement in the thermal conductivity and thus heat transfer in the heat exchanger by up to 10% and, moreover, as a result of the absorption process, a suspension which is immediately further processed in fertilizer and thus easily usable produ enabled.

As a heat transfer medium, sodium sulfate Dckahydrat (Na2S04 x 10 H) can be used, which is heated to a temperature of at least 33 degrees C by the magnesium hydroxide suspension, whereby the flue gases countercurrent to the circulating suspension and thereby cooled to a temperature, which is slightly below the temperature corresponding to the water vapor dew point, making use of the property of the sodium sulfate decahydrate to dissolve heat of dissolution in its own water of crystallization when heated above the - relatively low - temperature of 32 ° C and thus convert from crystalline to the liquid form, so that it is without difficulty via pipes to a filling station and - after filling in tanks or other transport containers can be brought to the consumer. Due to high heat of crystallization, increased by the thermally permissible overheating, the latent heat transfer medium used according to the invention has a high heat capacity, which exceeds the heat capacity of, for example, the temperature range in question by a multiple and which can also be released without problems at the destination by the consumer. Another advantage the invention also consists in the fact that energy supply can be largely adapted to the energy requirement, that the method of the invention ensures an extremely rational use of energy.

The flue gases are advantageously conducted in countercurrent to the circulating suspension and are cooled to a temperature slightly below the temperature corresponding to the water vapor dew point, with a stoichiometric excess of magnesium hydroxide of up to 2 in relation to the amount of flue gas pollutant preferably 1.25-1.50: 1 is constantly maintained. A 15% magnesium hydroxide solution is expediently used here. By guiding the flue gases and the absorption liquid in countercurrent, the flue gases - which are hotter than the magnesium hydroxide - initially absorb moisture, which later on when the flue gas cools down is deposited as a fine mist under the water vapor dew, thus simultaneously returning the heat of vaporization to the system contributes significantly to the separation of sulfur dioxide and nitrogen oxide impurities. Cleaning values of 90% and above are achieved in this way.

ΓÜRE

OMPI In order to maintain the cleaning power of the circulating suspension, a certain portion, amounting to about 10% of the total amount, is expediently continuously subtracted from the suspension and the amount remaining in circulation in return by adding fris suspension to the prescribed magnesium hydroxide excess and sodium hydroxide Hydrogen sulfate content is set.

In order to work up the suspension containing the pollutants, the suspension which has been removed can be concentrated with the aid of the heat contained in the flue gas to a magnesium hydroxide concentration of about 40% by weight and then alkalized with ammonia, whereupon the magnesium hydroxide which has been separated out in the alkalizing agent is removed from the solution by filtration . The ammonium lumitrite and ammonium sulfate remaining in the solution can either be used immediately as liquid fertilizer, or spray drying can be further processed into mixed fertilizer or pelletized with lime or other fertilizers. However, in a wide embodiment, the constituents contained in the suspension can be fractional crystallization into their sulfite / sulfate, Ni. rit / NLtrat-phosphate fractions are separated, the sulfur- or. Nitric acid and magnesium oxide can be further processed upon recycling of the phosphate fraction.

The method according to the invention is explained below by way of example using the attached block diagram.

In a coal-fired power plant with a capacity of 700 MW, about 5.5 fall

OMPI Bm ³ / h flue gases with a temperature of 120-140 ° C, which contain an average amount of substances of approx. 650 mg S02 and 400 mg NOx per Bm ³ . If one assumes a temperature difference of 60 ° C between the inlet and outlet temperatures of the gas to be treated, a recoverable heat of approx. 100 million kcal / h can be expected. The flue gas is passed to wash towers 1 of a total of 240 m ^{2 of} flow area in countercurrent to an essential sorption suspension (AS) formed by a 15% magnesium hydroxide solution, which absorbs the heat and SO 2 and NOx pollutants from the flue gas. The flow velocity of the flue gas b is approx. 5 m / sec, the residence time in the wash tower is approx. 5 sec and the water density is approx. 5 m ³ absorption slonLons per m ¹ . 1 m ³ / h absorption suspension (AS) are kept in circulation. At the beginning of the cycle, a 15% Mg (0H) 2 solution is prepared as an absorption suspension, which

Water . : approx. 1100.0 t NaH2P04: approx. 192.0 t

Mg (0H) 2: contains approx. 13.3 t. This absorption suspension (AS) is kept in circulation both in the heat-pollutant absorption stage and in the heat exchanger stage with an output of 1200 m ³ / h in countercurrent to the flue gas, where 13.3 t / h Mg (0H ) 2 and approx. 700 Nm3 / h are added. There is no discharge at the start of operation, the system only works in circulation mode in this phase.

The addition of N02 is intended to establish an equilibrium state in such a way that equimolar amounts of NO and N02 are present in the flue gas at all times, which is a prerequisite for the course of the reaction after the equation

OMPI 1) Mg (0H) 2 + NO + N02 Mg (N02) 2 + 1120

2) 2Mg (OH) 2 + 4 N02 Mg (N03) 2 + Mg (N02) 2 + 2 H20

3) Mg (0H) 2 + S02 MgS03 + H20

4) Mg (0H) 2 + S03 is MgS04 + H20. Overall, the implementation of the pollutants contained in the flue gas results in the following quantity balance as described:

Mg (0H) 2 insert 13.2644 t / h

MgS03 / S04 production 16.6667 t / h Mg (N03) "production 2.4474 t / h

• Mg (N02) 2 accumulation 3.9627 t / h total 23.0768 t / h.

The absorption suspension is concentrated according to the table shown below:

Working hours Concentration of pollutants Amount of AS,% by weight in the discharge (10% of the solids content in the discharge N Nuullllssttuunnddee 1 155,, 0000 - no discharge

1 16.80 1.95 no discharge

12 27.65 14.97 no discharge

24 32.00 20.24 no discharge

36 33.60 22.10 no order

48 34.15 22.75 no discharge

The table shows that after the 48-hour working cycle, the absorption _ a _

suspension has reached a concentration of approx. 35% by weight and that the m 100 m ³ / h of absorption suspension discharged amount of 22, t / h, ie approx. 23.0 t / h, almost corresponds to the amount of pollutants to be absorbed . At the beginning of the 49th working hour, 100 m ^{3 of} absorption suspension (approx. 35% by weight) per hour can then be drawn off for work-up (discharge) and 80 m ³ / h of water and 16 t / h of NalI2P04 can be added. 13.3 t / h Mg (0H) 2 are continuously added in the first hour.

The addition of sodium dihydrogen phosphate leads to a substantial increase in the thermal conductivity of the absorption suspension. Assumes that after 48 hours the suspension contains 0.290 mol / l magnesium hydroxide, 1.444 mol / l sodium dihydrophosphate, 0.121 mol / l magnesium sulfite / sulfate, 0.0 mol / l magnesium nitrate and 0.028 mol / l magnesium lumnitrite. This results in the following factors for the ions contained in the solution:

Ion ion conc. ci coefficient ai (mol / l)

Mg 0.553 - 0.0080 - 0.004424

0H 0.780 0.0180 + 0.014040

Na 1,444 0.00 0.00

H2P04 1.444 0.0180 + 0.025992

S03 0.121-0.0020-0.000242

N03 0.028 - 0.0060 - 0.000168

N02 0.056 - 0.0040 - 0.000224

OMPI Starting from a thermal conductivity of water at 100 ° C of 0, the addition of the ci * ai factors results in a thermal conductivity for the suspension which, when sodium dihydrophosphate is added, increases by 6%, but such an addition only by 1.5%. is above the thermal conductivity of the water. A further significant increase to over 10% can be achieved by increasing the sodium dihydrophosphate concentration or due to the low degree of dissociation of the magnesium hydroxide.

The Absorpticns suspension transfers the absorbed heat in a heat exchanger 2 to a circulating oil flow, which in turn releases the heat in the collector to a suitable latent heat store, for example Glauber's salt, for repeated heat absorption, which is then transported back to the location. In this way, you can save approx. 75 million kcal / h net. The heat transported with the latent heat storage consumer is fed either directly or indirectly via a heat pump 7 to the space heating 8 or the hot water heating

The flue gas to be cleaned is cooled down in the absorption stage to the limit of water and acid dew point (approx. 65-70 ° (.), In which areas there are particularly favorable absorption conditions and water evaporation is prevented, which would be equivalent to a loss of content. Due to work the countercurrent principle results in a total heating of the absorption suspension to a technically usable temperature of about 90 95 ° C. The cleaned flue gas is mixed with a fan 5 to reduce the relative humidity when venting into the atmosphere about 10% by volume of false air.

OMPI In one embodiment, the absorption suspension can be saturated with 6.9 t of ammonia / h for the purpose of further working up, Mg (0) being excreted and ammonium-sulfile / sulfates and ammonium-nitrite / nitrates being formed

18.8 t / h (NH4) 2S03 / (NH4) 2S04 6.9 t / h NH4N02 / NH4N03 16.0 t / h NaH2P04 (possibly mixed with ammonium phosphate),

i.e. a concentrated, approximately 40% liquid mixed fertilizer solution, which can be filtered off and recycled to the absorption stage (Mg (0H) 2) as a ready-to-use fertilizer product (approx. 100 mVh.). In a variant of this embodiment, the concentrated solution obtained can be spray dried, giving 41.7 t / h of mixed fertilizer in powder form. Instead of spray drying, drying can also be carried out by adding 43.6 t / h of additional material (e.g. lime) and thus producing a granulated mixed fertilizer that accumulates in an amount of 85.0 t / h.

In another preferred embodiment, the discharge from an evaporator 3 can be fractionated into a phosphate fraction, sulfite / sulfate fraction and a nitrite / nitrate fraction, from which the phosphate fraction is recycled, the sulfite / sulfate fraction to about 130 tt / a sulfuric acid ( 100%) and the nitrite / nitrate fraction to 350 t / a nitric acid (100%) are worked up.

Claims

1. A method for recovering the waste heat from the flue gases of V combustion systems, characterized in that the heat contained in the flue gas is initially set in a first stage by direct contact to a pH of 4.5 to 5.5 and on this pH value containing magnesium hydroxide suspension (Mg (0H) 2) and then indirectly transferred to a heat transfer medium in a heat exchange stage.

2. The method according to claim 1, characterized in that pH adjustment with the aid of sodium dihydrogen sulfate (NaH2P04) is the magnesium hydroxide suspension in an amount of 140 to 180, vorte liable 150 to 160 g / 1 is added.

3. The method according to claim 1 or 2, characterized in that heat transfer medium salt hydrates, for example Glauber's salt (Na2S04 x 10 H2 Mg (N03) / MgC12 x 6 H20, A12 (S04) 3 x 12 H20 or the like. As latent heat storage is used are preheated to a temperature close to the transition temperature by the magnesium hydroxide suspension.

4. The method according to claim 1, characterized in that the flue gases countercurrent to the circulating suspension and thereby cooled to a temperature which is slightly below the temperature corresponding to the water vapor dew point.

5. The method according to any one of claims 1 to 3, characterized in that the suspension contains about 15 wt.% Mg (0H) 2 and at a temperature of at least 32 degrees C.

6. The method according to claim 1, characterized in that the heat transfer medium is kept in circulation and the Wä stored in it is transferred to a latent heat storage.

7. The method according to claim 1, characterized in that the sprinkling density of the magnesium hydroxide suspension about 5m ³ / m ³ and

The residence time of the flue gas in contact with the suspension is approx. 5 seconds.

8. The method according to any one of claims 1 to 7, characterized in the suspension a - based on the amount of pollutant in the flue gas - stoichiometric excess of magnesium hydroxide of up to 2: 1, preferably 1.25-1.50: 1 constantly upright is obtained.

9. The method according to claim 1, characterized in that the flue gas entry into the absorption stage continuously nitrogen dioxide in egg

Amount is supplied which corresponds stoichiometrically to the amount of nitrogen monoxide in the upper part of the absorption column.

10. The method according to any one of claims 1 to 9, characterized in that a certain proportion of the circulating suspension is continuously deducted from processing and the amount remaining in circulation in return you - 1.3 -

Adding fresh suspension to the specified magnesium hydroxide overs and sodium hydrogen phosphate content is adjusted.

11. The method according to claim 10, characterized in that the portion of the absorption suspension withdrawn for processing is concentrated to a magnesi hydroxide concentration of 40% by weight and alkalized with ammonia, whereupon the magnesium hydroxide which has been separated is filtered off

12. The method according to claim 11, characterized in that the solution is spray-dried a water-dry compound fertilizer.

13. The method according to claim 1.1, characterized in that the solution is granulated by adding additives to mixed fertilizers.

14. The method according to claim 10, characterized in that the supens is supplied for direct use as a liquid fertilizer.

15. The method according to claim 10, characterized in that the solution is crystallized by fractional evaporative crystallization to separate nitrite / nitrate sulfite / sulfate and phosphate fractions.

16. The method according to claim 14, characterized in that the S fit / sulfate and nitrite / nitrate fractions to sulfur or. Nitrates can be processed further.

17. The method according to claim 14, characterized in that the phosph fraction is recycled.

OMPI